<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>高亮显示案例</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        ul,li{
            list-style: none;
        }
        .ul_index div{
            width: 300px;
            height: 300px;
            margin: 10px;
            background-color: red;
            opacity: 0.6;
        }
        li{
            background-color: black;
        }
    </style>
</head>
<body>
<ul id="ul" class="ul_index" style="display: flex; flex-flow: row wrap">
    <li><div></div></li>
    <li><div></div></li>
    <li><div></div></li>
    <li><div></div></li>
    <li><div></div></li>
    <li><div></div></li>
</ul>
</body>
<script src="jquery-3.3.1.min.js"></script>
<script>
    $(function () {
        $('#ul > li').mouseenter(function () {
            $(this).children('div').css('opacity',1);
        });
        $('#ul > li').mouseleave(function () {
            $(this).children('div').css('opacity',0.6);
        })
    });
</script>
</html>